1) Have you tried re-formatting the card in the dashcam ie, not in the computer?
2) Have you set the dashcam to loop ie, start overwriting the earliest records once the card is full, so that you only get the last journeys?
3) Have you cleared the "saved" files? Depending how you've set the sensitivity some files are saved to a separate area of the card which is not overwritten. This is to protect those files in case of accident. You have to delet those manually.
4) What sort of card have you got ie, make and size in GB? On my NextBase I had problems with Sandisk, known to be a good make and which I use in all my cameras. I returned one card to Sandisk under its lifetime warranty and they replaced the card but pointed out that for dashcam use you should use "High-Endurance" cards NOT "High-Performance".
5) Have you tried removing the card from the dashcam and putting it directly into your computer? (You sound like you connected the dashcam to the computer and tried the delete that way).